I think Dark Sun makes the most sense since it is by far the most different, mechanically speaking. The other settings can be very easily replicated with the rules as they exist. Greyhawk and Dragonlance would require no major changes, and only a handful of minor ones.
Ebberon would need a few races (a couple of which they already addressed in Unearthed Arcanna, if I recall correctly), a way to handle the dragonmarks, and some rules for the airships and such. Probably a few more minor changes, but nothing that seems beyond the rules we have and the options in the DMG.
Dark Sun needs races, significant alterations to certain core classes, a prominent and robust psionics system, and changes to magic and spells and how they work. Of them all, Dark Sun is the most different both mechanically and thematically. To me, it makes the most sense to go with that setting.
Personally, I'd prefer Planescape above them all, but I think Planescape is actually already implied by default. I have no doubt that we'll continue to see more Planescape elements show up in products. I don't know if we'll see a specifically Planescape product until after more worlds have been established.